1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a transfer device employed in the image forming apparatus.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
In recent years, a variety of recording media sheets such as paper having a leather-like texture and Japanese paper known as “Washi” have come on the market. Such recording media sheets have a coarse surface acquired through embossing. However, toner does not transfer well to such embossed surfaces of the recording media, in particular, recessed portions of the surface. This inadequate transfer of the toner appears as a pattern of light and dark patches in the resulting output image.
In order to overcome such difficulty, a superimposed bias, in which an alternating current (AC) voltage is superimposed on a direct current (DC) voltage, is supplied as a secondary transfer bias to enhance transferability of toner, as disclosed, for example, in JP-2006-267486-A.
In order to further enhance transferability of toner for recording media sheets having a coarse surface, the surface of the recording medium on which a toner image is transferred is supplied with a positive charge before transfer. During the transfer process, the transfer device is supplied with the superimposed bias as a transfer bias in which an AC voltage is superimposed on a DC voltage.
Although generally effective for its intended purpose, the toner is still not transferred well to the recessed portions of the surface of the recording medium, resulting in image defects such as multiple white spots or dropouts.
In view of the above, there is thus an unsolved need for an image forming apparatus capable of maintaining good toner transferability regardless of the type of recording media sheets.